By way of illustration, in my prior International Patent Application, PCT/GB98/00770, at least as amended on entry into the European Regional Phase, there is described and claimed:
A dispenser for a gaseous, gas borne or droplet substance, the dispenser including:                a body having a mouthpiece with an inhalation/insufflation orifice at its end;        a junction in the body for a source of gas or evaporable liquid comprising or containing the said substance (the source being carried by the body); and        a breath actuable valve, for controlling the release of said gas or liquid, comprising:                    a valve inlet connected to the junction;            a valve outlet;            a flexible tube extending from the junction, between the inlet and the outlet, for receiving the said gas or liquid, the tube having a portion which is movable between a closed position in which the tube is kinked for closure of the valve and an open position in which the tube is un-kinked for opening of the valve; and            a movable member, for moving the movable portion of the tube to control its kinking, and being movably mounted in the body for movement by the act of inhalation from a rest position towards the orifice—or at least in the direction of air flow through the dispenser;                        the tube being kinked to an obturating extent when the movable member is in a rest position and un-kinked when the movable member is moved on inhalation for release of the gas or liquid.        
The source of such a dispenser will typically be a pressurised can, having a valve for metering the dose of substance to be dispensed on each inhalation; although it is conceivable that this or a similar dispenser will dispense a dose which is not metered by the source, but rather is metered by the dispenser by filling of the duct between the source and the dispenser's valve on release of substance from the source.
There has been increasing interest in counting the number of doses dispensed from the source, for instance to alert the user to the number of doses which have been dispensed or more usually of the number of doses of known composition or strength which can still be dispensed.